This invention relates generally to the type of fuse that provides a visual indication when fusing occurs, and more specifically, this invention relates to the type of fuse that closes an alarm circuit when fusing occurs, while also providing a visual indication that the fuse has cleared.
In the present invention, a fuse is utilized in which a resilient or flexible spring contact is held in a flexed position by an electrically conducting fusible link connected between the spring contact and a stationary terminal. Upon melting or fusing of the fusible link, the resilient spring contact moves to engage a contact on another terminal within the fuse holder to complete an alarm circuit. An indicating device is carried on the end of the flexible spring contact to provide a visual indication that the fuse has cleared. A fuse of this type is disclosed in U.S. Pat. No. 3,190,987--Fister, issued on June 22, 1965 for "Protectors for Electric Circuit" and assigned to the same assignee as the present application.
Fuses of the type to which the present invention is directed are frequently employed in situations where a relatively large number of fuses are mounted in close proximity. In such an arrangement, provision of the visual indicator is particularly important, as it permits an operator to readily ascertain which of the fuses has cleared. An arrangement for mounting a multiplicity of such indicating fuses is disclosed in U.S. Pat. No. 3,538,480--Urani, issued on Nov. 3, 1970 for "Protectors for Electric Circuits" and assigned to the same assignee as the present application.
While these prior art indicating fuses have been used quite successfully, there are factors that have prevented optimum utilization of these indicating fuses. Thus, for example, the present indicating fuses may be utilized for a range of actuating or fusing currents, depending upon the size of the fusible link employed. However, there are situations in which it would be desirable to be able to obtain actuation of the fuse at very low current levels.
With present indicating fuses, there is a minimum level of fusing current imposed by the physical structure of the fuse because the flexible spring element provides a certain return spring force, depending upon the dimensions and material of the flexible spring contact, when it is flexed into the normal conducting position where it is held by the fusible link. However, if the size of the fusible link is decreased to too great an extent (i.e., constructed to sense and fuse in the presence of a very low current), the fusible link will be too limited in mechanical strength to be able to maintain the flexible contact in the flexed position. It is possible to overcome this problem by varying the size of the flexible spring contact, but it is highly desirable to be able to achieve sensitivity to a very low fusing current without modifying the basic structure of the indicating fuse.
Another potential problem involves the engagement between the spring contact terminal and the stationary terminal, on the one hand, and their corresponding terminal clips on the fuse holder with which they must provide a continuous electrical current path, on the other hand. Both the spring contact terminal and the stationary terminal are wrapped on extended ear portions of the insulating body of the fuse carrier, thereby taking on a generally closed U-shape. The terminal clips on the fuse holder have flexible members which forcibly engage the U-shaped terminals, as illustrated in FIGS. 11 and 12 of the above-identified Fister patent.
The generally U-shaped terminals have inwardly projecting flanges at the open ends of the U-shaped members, with the ends of the flanges having a gap of a predetermined width therebetween. This gap width is set such that the flanges can secure the generally U-shaped terminals in position. In order to give the necessary strength, and yet provide sufficient resiliency to permit the terminals to be placed on the insulating body during manufacture, the terminals are currently formed with a dimension across the bottom of the "U" which is somewhat greater than the corresponding dimension of the portion of the insulating body on which it is fitted. The result is that the legs of the U-shaped terminal and flanges pinch toward one another and the terminal ends up with a tapered configuration.
This tapered configuration is essentially only a problem upon engagement with the spring members of the corresponding terminal clips. As a result of the tapered configuration of the terminals, the spring portions of the terminal clips provide a force component that tends to drive the terminals out of engagement with the spring portions of the terminal clips. Accordingly, if the fuse is jostled or jiggled, the electrical engagement between the terminals and the terminal clips may be broken.
Also, the tapered structure of the terminals can result in less area of engagement between the spring portions of the terminal clips and the tapered U-shaped terminal itself. The smaller contacting area that results is also a potential source of problems. Therefore, it would be desirable to have the generally U-shaped terminals maintain the legs of the "U" in parallel rather than tapering toward the open end.
Another area for improvement is in connection with the visual indicator. Normally, the indicator is just placed on the end of the flexible spring contact, with its axis aligned with the axis of the spring contact. Although this indicator does suffice for visual observation, it is frequently difficult to observe it as readily as might be desired. Accordingly, it would be desirable to have the indicator more visible.